This proposal describes the design and testing of a new type of sensor for the measurement of total cholesterol based on deesterification by cholesterol esterase, oxidation by cholesterol oxidase, and mediated electron transfer from reduced cholesterol oxidase to an electrode by novel redox systems. Because these redox systems have quite negative formal potentials, the sensors can be operated at a potential where common interferents, such as ascorbic acid and uric acid, are not detected. The proposed research plan for Phase I concerns the development of the cholesterol sensor through the following approach: (1) synthesis of a variety of new monomeric and polymeric redox systems which can serve as mediators in conjunction with cholesterol oxidase; (2) construction of cholesterol sensors based on cholesterol esterase, cholesterol oxidase, and these electron transfer mediators; (3) covalent attachment of cholesterol esterase and cholesterol oxidase to the mediator-containing polymers in order to further increase the interaction between the oxidase and the mediators, and to improve the long-term stability of the sensor; (4) optimization of the sensor with respect to the following: rapid response to cholesterol, insensitivity to common electroactive interferents, insensitivity to oxygen, resistance to fouling in biological fluids, and long sensor lifetime. During Phase II, this small sensor will be coupled with the necessary electronics to produce a portable, self-contained device.